1. Field of the Invention
The present invention relates to a CCD type solid state image pick-up device and, particularly, to a CCD solid state image pick-up device in which charge transfer electrodes of a vertical and horizontal charge transfer portions thereof are formed in a single layer structure.
2. Description of the Related Art
Owing to the recent development of fine manufacturing techniques, it has become possible to form respective charge transfer electrodes of a CCD type solid state image pick-up device with fine spaces therebetween and, owing to this fact, it becomes possible to form a charge transfer device having a single electrode structure with inter-electrode distance or space being 0.2 to 0.3 .mu.m by etching an electrically conductive electrode material.
Since there is no overlapping portion between transfer electrodes of a single layer electrode structure of such charge transfer device, there are advantages that inter-electrode capacitance is small and that there is substantially no short-circuit occurs between electrodes. Further, since there is no need of the oxidation step for oxidizing electrode to form an inter-layer film necessary in the overlapping portions, there is another advantage that it is possible to use not only polysilicon but also metal film or its silicide film as electrode materials, with which it is possible to reduce the resistance of the charge transfer electrodes.
However, since, in the charge transfer device having the single layer electrode structure, the respective charge transfer electrodes are arranged with spaces determined by the preciseness of etching, there may be a region covered by the charge transfer electrodes and a region which is not covered thereby and potential well may be formed in the uncovered region, as shown in FIG. 10(a). The larger the depth of the potential well requires the larger the distance between adjacent electrodes as shown in FIG. 10(b). Further, the configuration and depth of the potential well depend upon variation of potential difference between charge transfer electrodes and, when the potential difference is large, the potential well becomes shallow due to the potential modulation effect as shown in FIG. 10(c).
In order to make the potential well shallower, it may be considered to regulate the potential in the region in which the potential well is formed by injecting ion into the region. FIGS. 11(a) and 11(b) show variations of the potential well when such ion injection is performed, in which depicts potential when ion injection is too small, depicts potential when ion injection is suitable and epicts potential when ion injection is excessive.
As is clear from FIGS. 11(a) and 11(b), when the amount of ion injected is too small, the effect of reducing the depth of the potential well is short, while, when the amount of ion injected is too much, there may be a potential barrier formed. Therefore, it is necessary to inject an optimal amount of ion into such region. However, the optimal amount of ion depends upon the distance between adjacent electrodes as mentioned above as well as an amplitude of a drive pulse applied to the electrodes. Therefore, in a case where the inter-electrode distance in the vertical charge transfer portion is different from that in the horizontal charge transfer portion or the amplitude of the drive pulse applied to the electrodes of the vertical charge transfer portion is different from that of the horizontal charge transfer portion, the optimal amount of ion injection in the vertical charge transfer portion becomes different from that in the horizontal charge transfer portion.
For example, when the amount of ion injection in the vertical charge transfer portion is optimized, the amount of ion injection in the horizontal charge transfer portion becomes short, so that the reducing effect of depth of the potential well becomes insufficient and a portion of signal charge may be left not transferred, while, the amount of ion injection in the horizontal charge transfer portion is optimized, the amount of ion injection in the vertical charge transfer portion becomes excessive, so that the potential barrier is formed.
Therefore, a charge transfer device which can sufficiently reduce the depth of potential well without forming the potential barrier in both the vertical and horizontal charge transfer portions has been highly desired.